A router device or router—a level three IP packet switch formerly called a gateway in the literature—contains a plurality of connections with other routers. These connections can be of different natures depending of the used protocol e.g. a point to point or a multi-access one, latter being used for an Ethernet based network. The use of a protocol like OSPF (Open Shortest Path First) protocol which is a TCP/IP (Transmission Control Protocol over Internet Protocol) will permit to the routers to get enough information about the network for a successful forwarding of the received packets.
The routing when using e.g. OSPF protocol will be dynamical i.e. a change in the topology of the network will be taken into account in a dynamic way. It implies that some exchange of information must be performed regularly between the routers so to update a database of each router concerning information of an actual status of the network.
It may be necessary according to the type of the connection and the used routing protocol to build a so-called adjacency process between several different routers. In that case, routers that become adjacent will have some information concerning the state of the network so to be able to forward any incoming packets. These information are organized in a routing table. And due to the dynamical mode, these routing tables must be updated regularly. But to optimize the traffic of packets, such adjacency process may be done only between a router and a designated neighbor router. The updates are performed via exchange of messages called Link State Advertisement LSA containing some information about the network. The routing tables are then calculating by each router using that information.
A lot of applications must be highly available. But due to potential hardware or software failures, desktop systems like routers are not, strictly speaking highly available. A way to improve the situation is to use a clustering model i.e. use a group of redundant and autonomous server platforms including routers in a networked configuration. In that case, if one autonomous platform fails, a standby platform will take its place. But such model has the big drawback that a takeover time is of the order of 30 to 90 seconds. During that takeover time, all packets will be lost which is an unsatisfactory situation for many application.
In U.S. Pat. No. 5,473,599 and in the documentation to be found on the Oct. 15, 2001 under http://www.cisco.com/univercd/cc/td/doc/product/lan/ cat6000/sft136131/configgd/redund.htm are explained an alternative to guarantee high availability using Hot Standby Router Protocol HSRP. In that case, a system database is maintained on an active supervisor engine (in our case a router) and updates are sent to a standby supervisor engine (an adjacent router) for any change of data in the system database. The active supervisor engine communicates and updates the standby supervisor engine when any state changes occur, ensuring that the standby supervisor engine knows the current protocol state of supported features. A synchronization of each database, among other the routing tables, must be performed. The standby supervisor engine is isolated from the system bus and does not switch packets. But it does receive packets from the switching bus to learn and populate the routing table. The standby supervisor engine does not participate in forwarding any packets and does not communicate with any modules.
When using HSRP, it may be convenient to use also Virtual Router Redundancy Protocol VRRP as discussed at the IETF and to be find on the Oct. 15, 2001 under http://www.ietf.org/html.charters/vrrp-charter.html. In that case, each element of the network in an HSRP group will share a virtual IP address and MAC (media access control) address. In other words, several routers on a LAN (Local Area Network) on a multi-access link will be able to use the same virtual IP address. One router will be elected as a master with the other routers acting as backups in case of the failure of the master router. The master will forward packets sent to these IP addresses. The election process provides dynamic fail over in the forwarding responsibility should the master become unavailable. This allows any of the virtual router IP addresses on the LAN to be used as the default first hop router by end-hosts. The advantage gained from using VRRP is a higher availability default path without requiring configuration of dynamic routing or router discovery protocols on every end-host. But it is based on the use of different routers on a LAN i.e. at least two, one for the master and one for the backup, and each of these routers being independent server platform, a very costly engine.